The invention relates to a method and system therefor for photodetection in which measurement signals in the form of signal pulses during a photon-counting operation are tapped off at a photomultiplier, the signal pulses being indicative of incident photons of the light being measured. Methods, systems and devices therefor which employ such photodetection schemes are particularly useful in chemical engineering, especially for performing highly sensitive analyses based on the principle of chemoluminescence or bioluminescence, such as luminescence immunoassays for example, reporter gene assays, or DNA assays. For such purposes, it is necessary to record the luminescence process, which occurs as a rule in the seconds range, with time resolution, with a high dynamic range being required for detecting the light quanta to investigate samples of an unknown concentration. In this connection, it is important to note that samples, as a rule, are investigated automatically at a high throughput rate and that they can only be measured once after the luminescence reaction has started.
To guarantee a high detection sensitivity, for example, in a known photodetection system for performing high sensitivity analyses based on the principle of chemoluminescence or bioluminescence, the photomultiplier that serves as the opto-receiver is employed in a so-called single-photon counting operation. In such operation, the photomultiplier provides sufficient amplification because of the presence of the selected high voltage and the many dynode stages to cause the generation of pulses of current or voltage that can effect the triggering of a single photoelectron. The voltage pulses thus produced can be distinguished from background noise by a discriminator. Depending on the quantum yield of the photocathode, therefore, individual incident photons can be detected. The disadvantage of this process, however, is that the bandwidth of known pulse amplifiers is insufficient at higher light intensities to resolve individual events in time without dead time losses. At higher photon rates, so-called pile-up effects cause problems, due to an overlapping of pulses.
It is also known in conjunction with the operation of photomultipliers to amplify the output signals in analog fashion in the so-called current mode, in which the detection sensitivity is reduced by suitable adjustment of the operation voltage, although higher light intensities can still be measured. The disadvantage of this operating mode, in addition to a reduced sensitivity, lies particularly in the low long-term stability caused primarily by changes in the quantum efficiency of the photocathode and multiplication fluctuations that result from dynode fatigue and hysteresis.